Laser diode interferometry is well known and well documented in the literature, using either classical interferometry or self-mixing (or optical feedback) interferometry. Although self-mixing interferometry is an attractive technique (the interference phenomena occurs inside the laser diode cavity), it is difficult to be combined with frequency stabilization for deriving a precise wavelength, since the emitting wavelength is affected by the optical feedback.
Nowadays, laser diodes are built as coherent sources which are integrated in several consumer products (e.g. CD and DVD players, optical mice, laser pointers). However, in order to achieve high-accuracy displacement measurements (with ppm or even sub-ppm accuracies) and long range measurement an absolute frequency stabilisation of the laser diode is mandatory. Furthermore the coherence length of the laser source has to be comparable to the measuring range.
Research and development on stabilised laser diode interferometers has been the subject of several research projects. In 1995, a consortium composed of the Observatoire Cantonal de Neuchâtel, the Institute of Microtechnology of Neuchâtel (IMT), and the company TESA could achieve absolute frequency stabilisation of laser diodes with respect to rubidium absorption lines for a high precision industrial interferometer in the framework of a CERS. Nevertheless, standard Fabry-Perot laser diodes (which were the most common laser diodes in those days) are affected by frequency mode hops (i.e. the frequency jumps by several tenths of nanometer and becomes out of the locking range of the stabilisation loop). This problem prevented the industrialisation of interferometer with such diodes.
The problem of mode hopping may be solved by using Distributed Bragg Reflector (DBR) or Distributed Feedback (DFB) laser diodes. The Bragg grating which is used in these technologies acts as frequency-selective mirror and allows to increase drastically the mode-hop free tuning range. DFB laser diodes emitting around 1.5 μm became the standard lasers used for telecommunication applications and are thus cost effective. However, the cost of so-called DBR and DFB (edge-emitting) lasers around 780 nm or 850 nm (corresponding to rubidium and caesium absorption wavelengths, respectively) is still high. Thanks to recent research projects in the field of miniature atomic clock, new technologies (such as Discrete Mode Diode lasers) and new suppliers are now emerging, thus indicating that more improvements can be expected in terms of performance.
Since 2004, Vertical Cavity Surface Emitting Laser Diodes (VCSELSs) are now manufactured in mass quantity (mainly for computer mice). They are composed of at least one Bragg grating and the length of the laser cavity is so small that any mode-hop is impossible unless the light is accidentally retro-reflected in the laser cavity. For instance, VCSELs used in laser computer mice are single-mode, emit around 850 nm (corresponding to Cs absorption wavelength), and require a very low threshold current. The drawbacks are however their coherence length which ranges from 1 m to only 3 m, the wavelength tolerance (+/−10 nm) and their frequency noise spectrum, which has a quite strong impact on interferometric phase fluctuations.
The generally limited coherence length of laser sources restricts the distance range of common interferometers. If the optical path length between reference arm and measuring arm is larger than the coherence length, the interference signal becomes very weak. Measurements far beyond the coherence length limit are not possible with standard detectors.
Furthermore, common interferometers may be sensitive to optical feedback from the reference and measurement arm. As a result, erratic mode-hops or even chaotic behavior of the laser radiation may result. Optical isolators are often used in order to mitigate this effect.
Moreover, the laser wavelength exhibits low-frequency variations. These often show up as 1/f-noise in the frequency spectrum. A relative change of the wavelength results in a corresponding change of the relative distance measured. The wavelength variations can be reduced by tuning the emitted laser wavelength to some frequency standard using temperature or current control of the laser. Depending on the control-loop bandwidth, a considerable portion of the 1/f-noise contributions can thereby be eliminated.
Moreover, the laser exhibits rapid fluctuations of the wavelength. These high-frequency fluctuations can be well described by (white) frequency noise that extends the acquisition bandwidth achievable with standard electronic components. On one hand, these frequency fluctuations cause phase-fluctuations which result in corresponding distance fluctuations. These effects can often be mitigated by proper filtering of the resulting distances. If such phase fluctuations are larger than 2π within the detection time, the determination of the average phase and therefore of the average distance fails. In this case, the acquisition electronics is not able to follow the signal, such that the phase can not be determined unambiguously and phase-unwrapping fails.
Above mentioned disadvantages of laser diodes, in particular VCSELs, in combination with interferometers for long range measurement of distances or changes of distance to a target may be avoided by using laser diodes with great coherence lengths or by using other kind of beam sources like gas lasers, e.g. HeNe-Lasers. Such a device (laser tracker) with a laser diode with great coherence length is described e.g. in European patent application No. 11187614.0. Additionally, state of the art total stations are typically equipped with gas lasers for interferometry. However, the arrangement of a gas laser for use of distance measurement requires quite some space and the costs of a specified laser diode with great coherence length or the costs for a gas laser source are comparably high. In particular, considering miniaturisation efforts, the space requirement are one main disadvantage for such devices.
Some embodiments of the present invention may provide for an improved interferometric measuring device with beam source comprising compact design and providing precise distance change measurements at long range distances, e.g. up to 100 m.
Some embodiments of the present invention may provide for an improved interferometric measuring device with a laser diode, the diode comprising relatively moderate coherence but the device anyway providing precise long range distance change measurement.
Some embodiments of the present invention may provide for an improved interferometer with a low cost and a comparable low performance laser diode and with compensation capability enabling long range distance change measurement.
Some embodiments of the invention relates to a method for determining a change of distance to an object by interferometry with emitting measurement laser light from a laser diode, receiving at least a part of the measurement laser light reflected from the object, superimposing the reflected measurement laser light with a reference laser light and thereby providing at least an interferometric phase and determining the change of distance to the object depending on the superimposition.
According to some embodiments of the invention, the measurement laser light is emitted with low coherence and broad spectral bandwidth, wherein an emitting wavelength of the measurement laser light is fluctuating hop-freely within the spectral bandwidth causing interferometric phase fluctuations, in particular wherein the measurement laser light is emitted with a coherence length between one and three meters. The interferometric phase is continuously detected with a first detection rate, the first detection rate and a rate for processing of the detected interferometric phase being that high that the interferometric phase fluctuations are continuously incrementally tracked so that successive interferometric phase states provided by successive detections of the interferometric phase differ by a phase shift of less than π, in particular by a fraction of π. Moreover, detected phase fluctuations are averaged for a defined averaging time period and an averaged phase is derived. The change of distance to the object is derived with a second detection rate in dependency on the averaged phase, wherein the second detection rate is correlated with the averaging time period.
According to some embodiments of the invention, an effect of the present detection and determination of distance changes is that distance changes are measurable over a measuring range, which is significantly greater than the coherence length of the measuring light used for the detection.
In particular, the distance to the object is assumed as changing maximal with a predefined velocity of the object, in particular 10 m/s, while detecting the successive interferometric phase states. The maximum object-velocity is chosen so that the phase fluctuations and a phase shift caused by the movement of the object together are less than π.